1. Field of the Invention
This invention relates to a process for preparing a cellulosic market pulp, and particularly relates to a process for reducing the energy requirements associated with drying a cellulosic pulp slurry.
2. Description of Related Art
A variety of processes, both mechanical and chemical, are known for treating cellulosic materials, such as softwoods and hardwoods, to remove their lignin fraction and produce a cellulosic pulp suitable for making paper and related paper products. Included among the chemical processes are the well-known Kraft and sulfite pulping processes. In the Kraft pulping process, a cellulosic source such as wood chips is digested with an alkaline pulping liquor containing sodium hydroxide and sodium sulfide; while the sulfite process, as the name implies, employs a sulfurous acid solution of an alkali or alkaline earth metal sulfite to effect lignin removal. All known processes also generally rely on some type of post-digestion bleaching to obtain additional lignin removal, and increase the whiteness and brightness of the pulp. To produce a marketable pulp product having a sufficient whiteness and brightness, the lignin content of the pulp generally should be reduced to below about 10 weight percent.
After bleaching, the cellulosic pulp at a consistency anywhere between about 8 and 15 weight percent, is washed to remove residual digestion and bleaching chemicals. The bleached aqueous pulp slurry having up to about 15 weight percent pulp solids may be held in inventory pending further processing. The pulp slurry at this point is suitable for making paper.
Often, however, instead of making paper with the pulp, the slurry is dewatered and dried to a moisture content of less than about 15 weight percent, preferably between about 3 and 10 weight percent. Continuous sheet forming and drying can be accomplished, for example, generally using one of three different types of equipment: the cylinder, Fourdrinier (i.e., single wire), and twin-wire machines, to produce a solid pulp product which can be compressed and baled convenient for storage and shipment. When this market pulp is rehydrated and redispersed in water, the resulting pulp slurry can be used for making paper products.
Dewatering and drying an aqueous pulp slurry is, as one might expect, a very energy intensive process. The dilute aqueous slurry of cellulosic, e.g. wood, fiber is directed onto a traveling screen for an initial, gross separation of water from fiber. As the water flows through the screen openings, fiber is accumulated and retained on the screen surface to form a wet, fibrous mat. Additional water is subsequently removed from the mat by mechanical pressing.
Screening and pressing steps remove approximately 96% of the water initially present in the original slurry leaving a consolidated pulp mat containing approximately 70 to 55% water and correspondingly 30-45% oven dry fiber. Since a satisfactory market pulp should contain approximately only 5 to 15% water in relation to the dry fiber weight, such additional water removal is normally accomplished by means of thermal vaporization. For this purpose, the pulp mat generally is passed in intimate surface contact over a series of steam heated, rotating cylinders, such mat being pressed against the hot surface of each cylinder about a major portion of its circumferential arc.
In summary, the three steps which are typically used to form a final market pulp product from a cellulosic pulp slurry, such as a bleached wood pulp, all relate to the removal of water from the fiber mat or web. These include:
(1) depositing pulp upon a screen (or "wire") to form a mat or web of pulp fiber. This step, known in its initial stage as formation, is usually accomplished by passing an aqueous dispersion of a low concentration of pulp (e.g., 0.2% to 1% by weight solids) over the screen. This screen, assisted in certain situations by vacuum or suction, increases the consistency of the mat or web to approximately 18 to 23 weight percent solids. Often during formation, a spray or shower of water also in directed onto the mat or web carried by the screen.
(2) compressing or squeezing the mat or web in a "press section" to remove additional water. This is usually accomplished by felt presses, a series of rollers each having a felted band for contact with the mat or web. These presses remove additional free water and some capillary water, thus resulting in an increase in consistency of the mat or web to a range of about 30 to 45 weight percent.
(3) drying the mat or web utilizing steam-heated equipment in a "dryer section." Here, the remaining water content of the mat or web is reduced to that desired for the final specific product, the consistency of which typically ranges between about 90 to 97 weight percent, more usually between 92 to 96 weight percent.
As mentioned above, the greatest energy use occurs during the thermal (steam-assisted) drying of the pulp product. Drying is a relatively expensive process, and the cost of drying is always a major part of the processing cost of the final pulp product. Anything that can be done to reduce the energy required to produce a dry, solid pulp product will have a significant impact on the overall economics of the pulping operation. The present invention relates to a process for reducing the energy requirements of the pulp drying process during the initial manufacturing of the pulp product.